N,N-Dihydrocarbylhydroxylamines, such as N,N-dialkylhydroxylamines and N,N-dibenzylhydroxylamine, are well known as useful stabilizers for a variety of polymeric substrates as is taught for example in U.S. Pat. Nos. 4,590,231; 4,668,721; 4,782,105 and 4,876,300.
A number of methods are known for the preparation of N,N-dialkylhydroxylamines, and are reviewed by S. R. Sandler and W. Karo in "Organic Function Group Preparations", Vol. 3, Academic Press, New York, 1972, Chapter 10. These methods include the direct oxidation of N,N-dialkylamines with aqueous hydrogen peroxide (U.S. Pat. No. 4,782,105); the metathesis reaction between an alkyl halide and a hydroxylamine in the presence of alkali; and the catalytic hydrogenation of nitrones (U.S. Pat. No. 4,910,340).
A method more germane to the instant invention is described by G. Vavon et al., Bull. Soc. Chim. France, 43, 231-237 (1928) wherein the catalytic hydrogenation of aldoximes using a platinum catalyst in wet ethanol in the presence of hydrochloric acid generates N,N-dialkylhydroxylamines via a reductive coupling reaction. Thus, heptaldehyde oxime formed N,N-diheptylhydroxylamine in excellent yield (90% unpurified), but benzaldehyde oxime gives a mixture of N,N-dibenzylhydroxylamine plus dibenzylamine.
The same reductive coupling of heptanal oxime or benzaldehyde oxime to N,N-diheptylhydroxylamine or N,N-dibenzylhydroxylamine is reported by R. F. Borch et al., J. Am. Chem. Soc., 93, 2897 (1971) using cyanohydridoborate anion as a selective reducing agent.
G. W. Gribble et al., Synthesis, 1977, 856 teach the preparation of N,N-dialkylhydroxylamine by a similar reductive coupling reaction using an aldehyde oxime with sodium borohydride in an acid media. N,N-Di-n-butylhydroxylamine is prepared by reacting butanal oxime with sodium borohydride in acetic acid.
It is noted that in each of the reductive coupling reactions described supra, coupling produces a half equivalent of hydroxylamine as a byproduct which is either reduced to ammonia under the reaction conditions or remains as a contaminant in the desired N,N-dialkylhydroxylamine main product.
The instant process overcomes this disadvantage and additionally provides several advantages of its own. The instant starting materials are readily available aldehydes and inexpensive readily available hydroxylamine hydrochloride. This allows for great flexibility in syntheses since a wide variety of aldehydes are accessible and there is no need to isolate any intermediates such as the oximes.
More importantly from an economic and environmental point of view, since an oxime is not a starting material, only one half equivalent of hydroxylamine hydrochloride relative to the aldehyde is really required by the overall stoichiometry of the process. ##STR1## Accordingly, the instant process can be carried out in such a way that no hydroxylamine byproduct is produced.